The present invention relates to an electronic ballast for starting and operating gaseous discharge lamps. More particularly, it relates to a means for operating and starting at least one gaseous discharge lamp utilizing solid-state circuitry and a feedback power supply for operating the accompanying control circuitry.
Gaseous discharge lamps, for example, fluorescent, sodium vapor, mercury, and others have a negative resistance characteristic. That is, as the current begins to rise in the lamp, the resistance across the lamp decreases thereby allowing more and more current to be drawn, eventually destroying the lamp. A means of limiting this current is, therefore, necessary to enable the lamp to operate at a sufficient intensity for proper illumination, yet, at such a current level so that the lamp will not run away with itself and thus be destroyed.
Various arrangements of inductors, transformers, capacitors, and resistors have been used to ballast gaseous discharge lamps. One of the most common devices used to ballast the lamp is an autotransformer-power capacitor arrangement connected in series with the lamp. One of the disadvantages of this conventional ballasting system is that peak energy stored in the reactive elements in the system must be maintained at relatively high levels to perform the ballasting functions and the relative rate of storage and release of this energy is low. Furthermore, large amounts of volt-amperes must be circulated in the system. These conventional ballasts are also quite heavy and bulky. Resistive ballasts have also been used, usually in DC systems, where the current is limited by a resistor placed in series with the lamp. This method, however, dissipates a large amount of power in the resistor and is, thus, very inefficient.
There also have been provided circuits for operating gaseous discharge lamps on a DC current utilizing electronic ballasting and control. One example of this approach is shown in U.S. Pat. No. 3,265,930 -- W. F. Powell, Jr., assigned to the General Electric Company, assignee of the present invention, which is hereby incorporated by reference. The Powell patent utilizes a time-ratio control device which includes a regulating transistor connected in series with a coasting inductor, a gaseous discharge lamp and a flyback diode. A sensing resistor is also connected in series with the regulating transistor and the lamp. The sensing resistor and the regulating transistor are connected to a control circuit for controlling the regulating transistor. Power which controls the regulation transistor is provided through a small transformer from the input line voltage, which results in power being continually provided for the control circuit even though the lamp is turned off.
It is desirable to utilize a power supply for the control circuit which energizes the control circuitry substantially only while the lamp is operating. Furthermore, it is impractical to operate some lamps, for example, sodium vapor, on DC. There also are applications where the input voltage available is too high to be utilized by the lamp. Therefore a means of stepping down this voltage is necessary, and this voltage should be useful in operating various types of lamps.
Some lamps, for example sodium vapor, have been known to break apart because of acoustic resonance which occurs when operated at certain frequencies. These frequencies will vary in different lamps. It is also, therefore, desirable to operate these lamps with a voltage wave form which overcomes the acoustic resonance problem.
The most common method of starting gaseous discharge lamps is the application of a high peak voltage across the lamp electrodes in order to begin the ionization process. However, in electronic ballast situations, this high voltage would require the use of high voltage semi-conductors and often high current semi-conductors which are either vary expensive or not available. It is, therefore, desirable to provide a starting circuit which overcomes these problems.
When operating an arc lamp, such as a linear metal halide lamp, in conjunction with a time-ratio control circuit, it is necessary that a transistor bridge inverter be provided that the lamp might be driven with a rectangular wave. The duty cycle of the bridge inverter is controlled in order to maintain proper spectralbalance throughout the length of the lamp. Furthermore, in order to prevent the power supply from being momentarily short circuited as one pair of transistors in the bridge inverter is turned off and the other pair turned on, it is necessary to provide a short period (5 - 15 microseconds) during which all four of the transistors are "off" or nonconducting. However, this momentary interruption of the load current will result in a voltage pulse being generated at the output of the time-ratio control ballast circuit. Such a transient pulse could possibly be of sufficient amplitude to destroy one or more of the transistors in the bridge inverter. It is desirable therefore, to provide a means of limiting the magnitude of this pulse to values which can be tolerated by the transistors.
Accordingly, it is one object of this invention to provide an electronic ballast circuit capable of starting and operating various types of gaseous discharge lamps.
Another object is to provide an electronic ballast utilizing a time-ratio control circuit and an inverter circuit for operating gaseous discharge lamps without the necessity of the normal bulky AC ballast or inefficient resistor ballast.
Another object is to provide an electronic ballast whose control circuitry is powered by a power supply which operates substantially only during the operation of the lamp.
Another object is to provide an electronic ballast utilizing a time-ratio control circuit and an inverter circuit and timing circuitry for operating at least one gaseous discharge lamp.
Another object is to provide an inverter circuit for operating at least one gaseous discharge lamp, the inverter circuit having transient protection for shunting transients back to the input.
Another object is to provide a time-ratio control circuit and inverter circuit for operating gaseous discharge lamps whereby the power for the control circuitry of the time-ratio control is provided by a regenerative feedback power supply.
Another object is to provide an electronic ballast for providing substantially square waves for operating at least one gaseous discharge lamp without acoustic resonance.
Another object is to provide an electronic ballast for operating at least one gaseous discharge lamp having a starting circuit which starts the lamp with a high voltage pulse effectively applied between one lamp electrode and a grounding plane.
Another object is to provide an electronic ballast for operating at least one gaseous discharge lamp at substantially constant wattage.